Field of the Invention
The present invention relates to an image printing apparatus using color materials and an image printing method. In particular, the present invention relates to a color material for obtaining a smooth color image with reduced graininess.
Description of the Related Art
A printing apparatus for printing an image through a subtractive process by a pseudo halftoning method uses color materials of three primary colors for printing: cyan, magenta, and yellow. By adjusting the numbers of dots of the three primary colors and a balance among the three primary colors, various densities and colors are represented on a paper surface. Further, many color printing apparatuses use a black color material in addition to the above-mentioned three primary colors to improve contrast and color stability in an achromatic color image.
In an image printing apparatus using the pseudo halftoning method, graininess has been one of the problems to be solved. Recently, graininess has been made less visible, for example, by using a print head capable of ejecting a small droplet ink to reduce the size of a dot on the paper surface or by using ink having a higher lightness than cyan and magenta, such as light cyan ink and light magenta ink. However, preparing additional ink such as light cyan and light magenta may lead to upsizing of the apparatus and an increase in running costs.
Japanese Patent Laid-Open No. 2002-154240 discloses a method for preparing light blue instead of light cyan and light magenta by taking into consideration the fact that graininess caused by overlaying cyan ink and magenta ink, in particular, tends to be a problem associated with an image. Using light blue as disclosed in Japanese Patent Laid-Open No. 2002-154240 can effectively reduce graininess in an image while limiting the types of additional color materials to one.
Japanese Patent Laid-Open No. 2002-154240 discloses setting the lightness of light blue ink higher than that of cyan ink and magenta ink, but it does not particularly explain other features. Accordingly, even if graininess in, for example, a highlight in a blue image is reduced, graininess sometimes becomes conspicuous in other areas depending on gradations and hues. In particular, in a case where there is a great difference in lightness between light blue ink and cyan ink or magenta ink, graininess may be worse in an area in which cyan dots and magenta dots or blue dots formed by overlaying the cyan dots and the magenta dots are sparsely printed in a light blue image area.